This invention relates to an oil well pipe casing which is not fractured, even when used in the circumstances where the pipe receives a strain caused by ground subsidence or the like.
An oil well for drawing crude oil from oil reservoir up to the surface is formed by an oil well pipe, and the oil well pipe is composed of an oil well pipe tubing which draws up the crude oil and an oil well pipe casing which protects the oil well pipe tubing at its exterior.
Since the oil well mentioned above is, in general, bored vertically, a frictional force acts on the oil well pipe casing (hereinafter, simply referred to as oil well pipe) in the downward direction, when ground subsidence occurs.
However, in most conventional oil wells, the frictional force acting on the aforementioned oil well pipe is not always so great. Accordingly, the probability of fracturing the oil well pipe is low, and a measure for reducing the frictional force has not been taken.
While, recently, the depth of oil wells tends to be deeper with worse oil lifting conditions of crude oil, oil well pipe fracture problems by ground subsidence have become greater. That is, by the great depth, the oil well pipe is loaded with a great pressure resulting in an increase in the frictional force acting thereon to break the oil well pipe, and now, such a situation cannot be neglected.
As a solution, U.S. Pat. No. 4,483,396 discloses an oil well pipe formed by combining two pipes having a different diameter. They move by sliding with respect to each other to vary the total length and absorb strain, and thereby, the influence of ground subsidence is reduced. However, the oil well pipe operates on a mechanical principle, and the cost of itself and peripheral equipment for operating it is very expensive. Furthermore, the oil well pipe is not effective for reducing strain on the pipe body and is effective only for reducing strain on the part to which the equipment is introduced, and therefore, it is not so practical.
The invention has been made in order to solve such a problem, and an object of the invention is to provide an oil well pipe which hardly fractures by ground subsidence and the like by providing a sliding layer on the outside of the body of the oil well pipe to reduce the frictional force acting on the oil well pipe.
Furthermore, in most cases, there is a long distance between the manufacturing place of oil well pipes and the place where they are actually used, and transportation of them is not so easy. Moreover, upon construction, it is necessary to set them up to reach a great depth. Therefore, the pipes are exposed to various external physical shocks before the practical use thereof. As a result of investigating damage to the sliding layers, the inventors found that there is a possibility of the sliding layer not exhibiting its own function by the partial separation of the sliding layer from the surface of a steel pipe.
Moreover, they also found a possibility of damaging the sliding layer during construction for burying the oil well pipe into the ground.
Furthermore, they also found that, taking the circumstance where the oil well pipe is left into consideration, the temperature of the oil well pipe becomes high, similar to the temperature during operation, during its construction, and the sliding layer begins to slide in the step of setting the oil well pipe.
Thereupon, a further object of the invention is to provide an oil well pipe capable of reducing the sliding phenomenon during construction by restricting the properties of the sliding layer, and furthermore, preventing the destruction of the sliding layer and degradation of frictional force-reducing effects by external physical shocks during transportation or construction of the oil well pipe by improving the adhesive force between the sliding layer and the oil well pipe body and introducing a protection layer on the sliding layer.
The present invention has been made in order to solve the above problems, and prevents an oil well pipe from fracturing by restricting the properties of the sliding layer, so that the sliding layer does not slide during construction and deforms with ground subsidence to reduce the frictional force.
Moreover, the present invention prevents damage due to physical shocks by providing an adhesive layer to strengthen the adhesion between the sliding layer and the steel pipe and by introducing a protection layer on the sliding layer.
The sliding layer covers the outside of the oil well pipe and exhibits a viscosity at the operating temperature. Accordingly, when frictional forces accompanied with ground subsidence acts on the oil well pipe buried vertically in the ground, the sliding layer having a viscosity and covering the outside of the oil well pipe is drawn downward and deformed by the frictional force. Since most of the frictional force acting on the oil well pipe is spent in the deformation of the sliding layer, the frictional force acting on the oil well pipe body is reduced.
Such a phenomenon is explained using a schematic view shown in FIG. 3 as follows: Namely, when shear stress caused by ground subsidence acts on the upper face of the sliding layer 10 h in thickness and exhibiting a viscous behavior, shear deformation (viscous behavior) as shown in the figure by a broken line occurs after t seconds in response to residual negative frictional force xcfx84.
The relation between the quantity of shear deformation d(t) after t seconds and the residual negative frictional force xcfx84 can be represented by the formula (1) using a stiffness factor S(t) of the sliding layer and thickness h of the sliding layer.
xcfx84=S(t)xc2x7d(t)/3hxe2x80x83xe2x80x83(1)
From the formula (1), it can be seen that the residual negative frictional force xcfx84 can be made small by using a material having a small stiffness factor S(t) as the sliding layer.
By a model experiment, it has been found that, in order to decrease the possibility of fracturing an oil well pipe by ground subsidence, the fracture can be inhibited completely by rendering the frictional force xcfx84 equal to 4xc3x97103 N/m2 or less, particularly 2xc3x97103 N/m2 or less.
Since the friction force xcfx84 depends on oil well temperature, thickness of the sliding layer, quantity of ground subsidence and time, the thickness of the sliding layer and the stiffness factor of the material of the sliding layer for rendering the frictional force xcfx84 equal to 2xc3x97103 N/m2 or less can be determined by substituting the above values under the practical operating conditions for the formula (1). By providing the oil well pipe with a sliding layer so designed, the fracture of the oil well pipe caused by ground subsidence at an oil well is prevented.
Furthermore, as the S(t) value, it was found to be enough to use the stiffness after 1 year determined by extrapolating a time-stiffness curve obtained based on the measurement of a dynamic viscoelasticity meter, a shear-creep tester, a sliding plate rheometer or the like.
In general, the thickness of the sliding layer is 1 mm or more, and the shear quantity for 1 year is 5 mm or more. Then, when the stiffness factor S(t) after 1 year is in the range of 10xe2x88x925 N/m2 to 103 N/m2, the frictional force l stays within 2xc3x97103 N/m2. As a result, the oil well pipe resists fracturing and the fracturing of the oil well does not substantially occur.
While, in the case of 10xe2x88x925 or less, it melts away.
In other words, under general conditions as above, materials having a stiffness factor S(t) exceeding 103 N/m2 after 1 year are insufficient in deformability, and the fracture probability of the oil well pipe is high without reducing the frictional force.
Namely, in the case where the sliding layer has a thickness of 2 mm and a quantity of shear d(t) after 1 year of 10 mm, when the stiffness factor S(t) after 1 year is 103 N/m2 or less, the frictional force xcfx84 stays within 2xc3x97103 N/m2, and the oil well pipe resists fracturing. Furthermore, when the stiffness factor S(t) after 1 year is 102 N/M2 or less, the frictional force acting on the sliding layer stays within 2xc3x97102 N/m2, and fracturing of the oil well pipe does not substantially occur.
Besides, it was found that, since the temperature of the oil well pipe upon construction is frequently lower than the operating temperature by about 20xc2x0 C., which is also a high temperature, deformation of the sliding layer begins by the weight of the sliding layer or various stresses occurring during construction. It was also found that, in order to avoid it, when the stiffness of the material after 10 days is 2xc3x97102 N/M2 or more, particularly 103 N/m2 in the temperature range of construction, the above shear deformation does not occur.
As mentioned above, the oil well pipe of the invention functions to reduce the frictional force between sunken ground and the oil well pipe body by the viscous behavior of the sliding layer to prevent the oil well pipe from fracturing. The thickness of the sliding layer is selected according to the temperature where the oil well pipe is used and the quantity of ground subsidence.